1. Technical Field
The present invention relates to a method for transmitting position signals and reference pulses, as well as warning signals indicating a malfunction state, from a position measuring arrangement for determining the position of two elements of a machine which are movable relative to each other, to an evaluation unit. The present invention further relates to a device for transmitting substantially rectangular-shaped position signals and reference pulses, as well as warning signals indicating a malfunction state, from a position measuring arrangement for determining the position of two elements of a machine which are movable relative to each other, to an evaluation unit.
2. Background Information
A position measuring arrangement is used for the determination of the position of two objects which can be moved in relation to each other, for example two machine parts of a machine tool which can be moved in relation to each other. A measurement representation, for example in the form of a scale graduation, is connected with the one of the two objects, and a scanning unit with the other, so that, by scanning the scale graduation, it becomes possible to determine the extent of the movement of the two objects with respect to each other along the scale graduation. In this case the generation of position signals by the position measuring arrangement takes place by scanning, for example optical scanning, of the scale graduation by a measured value sensor of the position measuring arrangement in the course of the relative movement of the scanning unit and the scale graduation. By the scanning unit on the one hand, and the scale graduation on the other being arranged on one of the two objects which are movable with respect to each other, it is possible to detect the respective position of the one object in relation to the other object, either in the form of an absolute position, when employing a scale graduation constituted by a code track, or in the form of relative position changes, when employing an incremental periodic scale graduation. In this case the generation of the different types of position signals can take place by employing optical, magnetic, inductive or capacitive scanning units.
When using incremental position measuring arrangements for position measuring, at least two periodic analog position signals, which are phase-shifted in relation to each other, are generated at the output side by scanning a periodic scale graduation by a scanning unit which, for determining the relative position of the scale graduation and the scanning unit, are evaluated in an evaluation unit including, for example, of a numerical control of a machine tool.
The incremental position signals generated by the position measuring arrangement are transmitted, depending on the type of transmission, via two or four signal transmission lines in a parallel form to the downstream arranged evaluation unit. In the case of incremental position signals, in connection with the relative movement of the scale graduation and the scanning unit, the position measuring arrangement provides two position signals phase-shifted by 90° in a single-phase transmission. In case of a differential transmission, an inverted position signal is additionally transmitted from the position measuring arrangement to the evaluation unit with each one of the two 90° phase-shifted position signals.
In addition to the position signals and possibly inverted position signals, reference pulses are transmitted from the position measuring arrangement to the evaluation unit, which are generated by logically interconnecting reference pulse signals, derived from the scale graduation, with the position signals. In case of single-phase transmission, a reference pulse is transmitted via a further signal transmission line to the evaluation unit and is further processed there, while in case of a differential transmission two further signal transmission lines are provided, through which a reference pulse and an inverted reference pulse are transmitted.
Besides the incremental position signals and the reference pulses, a warning signal can be output by the position measuring arrangement to the evaluation unit if, for example, permissible signal amplitudes fall below a threshold, or other events occur which endanger a dependable operation and are detected by a monitoring unit, which is connected with the position measuring arrangement or is integrated into the position measuring arrangement and outputs the warning signal. For transmitting the warning signal from the position measuring arrangement to the evaluation unit, a single-phase signal which, in case of the transmission of rectangular-shaped warning signals with logical high and low levels, assumes a high level in the normal state, for example, while the warning state corresponds to a low level state, is transmitted to the evaluation unit via an additional output of the position measuring arrangement, as well as via an additional warning signal transmission line.
The disadvantage of this type of transmission of a warning signal is the requirement for a separate warning signal transmission line, which goes counter to the demand to keep the number of required signal transmission lines between the position measuring arrangement and the downstream-arranged evaluation unit as low as possible. Besides avoiding unnecessary outlay for wiring, it is intended to assure, if possible, the compatibility with the customary number of signal transmission lines when transmitting possibly desired additional signals.
For issuing warning signals it is alternatively possible to switch all outputs of the position measuring arrangement to high resistivity so that in the warning state instead of the differential signals only signals of the same level are present, which is recognized by the electronic follow-up device of the evaluation unit as a malfunction state. This way of transmitting a warning signal has the advantage that the warning signal does not require its own warning signal transmission line, and that it is additionally also possible to detect a line break in the warning signal transmission line, because in that case a low level is present at the follow-up device of the evaluation unit, which corresponds to a warning state. A substantial disadvantage of this type of transmission of a warning signal lies in that the transmission of signals is interrupted in the warning state, and the machine equipped with the position measuring arrangement must be immediately stopped.